1. Field of the Invention
The invention relates generally to the field of computer systems. More particularly, the invention relates to computer systems in which the memory (RAM) or a portion thereof is not adjacent to the central processing unit (CPU).
2. Discussion of the Related Art
In a typical computing system, the RAM is essentially adjacent to the CPU, so that wide buses, high-speed paths, and simplistic control means are adequate to achieve excellent transfer performance for data between the RAM and the CPU.
For RAM not physically adjacent to the CPU, one known technology for providing efficient transfer is known as “synchronous coherent interface” (SCI). SCI describes a specific, rather complex set of protocols including directory-based and pointer-passing coherence schemes via a serial link which is suitable for the connection of non-adjacent RAM to a CPU.
A problem with this technology has been that SCI is rather more expensive and elaborate than what is needed in some cases. More importantly, SCI was intended for the connection of multiple CPUs to multiple RAMs, none necessarily adjacent, and to maintain full coherence in an environment where every CPU can access all of the RAM.
Because of this generality, SCI cannot achieve excellent or even good performance for systems where latency is the key performance parameter, and where much of the generality of SCI is not required.
Heretofore, the requirement of protocols and methods to efficiently and correctly process memory accesses to the non-adjacent portion of RAM has not been fully met. What is needed is a solution that addresses this requirement.